If you have metering running on all tracks, try turning it off on everything possible. It looks cool with everything bouncing around, but it eats up a lot of overhead.
Perhaps try archiving tracks that you don't immediately need, and the computer will only work on the tracks you have un-archived. For instance, mix down drums to stereo, then archive the original tracks. Mixdown others, archive originals. Eventually, you'll have fewer tracks running at once, which should help.
Make sure you have tweaked your computer and have your settings optimized.
You may have to do some guerilla techniques. Instead of trying to run all effects real-time, perhaps process some of them in-place and save them as another track so the track doesn't use more overhead. Archive the original in case you want to go back to it later. You may want to nuke the processed track, and do the original differently.
Anything to lessen the number-crunching will help on a near-maxed computer.
These are just a couple of early-morning immediate thoughts. There are probably more.

Kapt.Krunch

New computer

I just updated the computer. Now all the mixes run smoothly. But there's still one thing annoying all the way: when a mix is nearly done. it takes absolutely ages to make edits e.g. add a midi note, change volume envelope or copy/move a clip etc.
What is causing this? Is this typical to Sonar?
I had the Windows CPU meter running when making edits and it absolutely jumps through the ceiling when adding just a single midi note. 20sec later its done.
I'm currently mixing 50 tracks five of which are midi.
The setup: new mobo with 1,5gb ram, 3500 cpu, a dedicated audio drive, XP sp2 etc.

Have you tweaked your computer and turned off as much non-essential garbage and stuff that runs in the background as you possibly can? Check out these:

http://www.musicxp.net/

http://www.tweakxp.com/performance_tweaks.aspx

http://mywebpages.comcast.net/SupportCD/OptimizeXP.html

http://www.soundonsound.com/sos/Mar02/articles/pcmusician0302.asp

What are you using for MIDI playback? Soundcard? Virtual instruments? If virtual instruments, what kind, where do they play back from?

50 tracks of audio will eat up a lot of power. 50 tracks of audio with processor plug-ins will eat up more. Saving processed tracks, archiving, and submixing may help. When editing, say, a MIDI track, do you really need all 50 audio tracks playing? Could you just narrow it down to the essential and minimal tracks that will give you the framework, and then when you are finished editing, re-enable the rest of the tracks for mixing? If you have a lot of duplicated instruments playing the same basic thing, perhaps strip it down to a basic track or two? For instance, because I have no string orchestra, I'll copy tracks of the same basic MIDI performance and assign each to a different sound, and possibly play around with velocities, timings, effects, pannings, etc. to try to make them sound more lush when added together. I may have a snappy snare from a Roland module hitting at the same time as a more woody, rounder VSTi snare, and do a bit of velocity, filter type stuff on each to keep it from sounding quite so static. Heck I may have three snares doing this.

The point is, if the notes are basically the same on several instruments, and I've already audio-ized everything but wish to make some changes, then I'll enable a minimum of audio tracks to tweak MIDI. Less overhead while working.

I realize it's best to have everything working if you want to fit in and tweak the sound within all the tracks, but if you are adding notes, moving notes, etc., you can probably get a good idea from minimal tracks.

I have an old MIDI computer that is synced to a newer, high-power audio-only computer. I always try to get the basic structure of the MIDI piece finished before I start adding audio, or converting MIDI-generated stuff to audio. When I am satisfied with the basic MIDI performance through the dreaded SB Live!, then I'll fire up the audio computer, dump the MIDI file in, and start adding guitar and vocals, and possibly replacing organ and bass stuff with a real Hammond and real bass. I can then drop the MIDI organ and bass tracks, if I want.
What I do then is start trying to find sounds on MIDI modules, keyboards, VSTi's...even Soundfonts, that will mix and match around the audio stuff. I may have 3 snares, 2 kicks, 2 or 3 different crash cymbals, all tweaked as I go, but in an attempt to give the illusion that it's one snare, one kick, and one crash. I'm trying to find the sounds that fit around the audio before I turn the MIDI-generated stuff into audio. It's easier and less processor intensive to tweak it in MIDI than in audio, and generally less destructive than doing radical tweaks to audio.

The point is that it may be a workflow issue that is holding you up. I'm only guessing that a lot of those 50 tracks of audio started off as MIDI? Wait as long as possible to convert MIDI tracks to audio. Of course, if you have all your MIDI-generating modules, cards, etc., used up, and think you need more, then you have no choice but to dump some if it to audio to free up sounds for more MIDI. But you may want to wait until late in the game, and you pretty much have all your real-instrument audio in before you turn MIDI tracks into audio, and then start to add more.

It's probably a lot better and less destructive to futz with MIDI sounds before converting to audio, then to try to manipulate audio.

One other thing comes to mind. You do have your Sonar clock source set to "Audio", don't you? Anytime you have audio playing in a project, you don't want it set to "MIDI Sync" or "Internal". Sonar pretty much wants to be master and have everything follow audio. Also, you may want to make sure "Echo Mode" is set to "None". That can sometimes lead to weird things.

Just some stuff to consider.

Kapt.Krunch

Cheers!
I'll check out the links!
The midi is soundcard (SW1000gx) stuff.
Most of the audio is vocal bits and pieces. I admit it's a very vocal oriented stuff. Obviously I could do submixes every now and then but I need to have things fully in control. Sometimes I need to make big changes and I hate going back to squere two for that.
Not all the tracks are running simultaneoulsy, 30 is max. Muting tracks make no difference. It seems the envelopes slow things the most. Maybe I need to learn a new way of mixing, leaving the volume changes 'till the last day. Half of the tracks normally require volume envelope, some need even eq.
Can you change the resolution of the envelopes? I.e. they don't need to be in 65000 steps, a rougher scale would do just as well: sometimes you just need to do one 2dB change at one point.
There's plenty of plugins running but no soft synths.

Muting an audio track in Sonar will not reduce the CPU usage as muted tracks are still streamed to the audio processor. This is so you can Unmute during playback and hear the track.

To stop the unwanted tracks using CPU you MUST archive them as Kapt.Krunch said. They will no longer be available to unmute during playback but will NOT use up valuable CPU power.

With regard to the huge delays in editing MIDI etc. you may do better to search the Sonar forums as I have seen some comments about this in the past. I think this applied to some of the older versions of Sonar.