As far as I could work out the laggy phone application isn't related to the screen glitch. I had both.
I fixed the screen glitch last week using the fix by MohommadAG but the laggy phone application continued.
I counted 5 rings on the callers phone (when I tested) and others asked why so long to answer etc.

I finally bit the bullet and reflashed my n900 and now the phone responds on or milliseconds after the first ring on the callers side.

All in all it took about 20 minutes to flash and then use the restore applications feature and I'm back and going with the phone working properly.
I looked everywhere for a fix to this and flashing looks like the only answer.