I've experienced that Thunderbird has gotten slower and slower on my Laptop, at last it was nearly unusable. Sometimes I could see the chars appearing slowly on the screen while I typed the email or it was impossible to drag & drop a email to a different folder because Thunderbird was stalled for 10-15 seconds. No other applications on my System were slow and the notebook has afast harddisk and SSD, so this was not the problem. The following steps helped me to speedup Thunderbird so that it got usable again.
Set some interface options
Edit Thunderbird options under Preferences → Advanced → General tab, click on "Advanced options" button and set these values:
layers.acceleration.disabled = true
gfx.direct2d.disabled = false
and restart Thunderbird. If it is still slow, try the next options:
Disabling the AeroGlass interface makes the interface reacting much faster. Install the "NoGlass" Addon which is available in the Thunderbird addon repository.
Disable Folder Indexing
If you have folders with many emails inside, Indexing can slow down Thunderbird. Go To Preferences → Advanced → General tab and disable the Global search.
Antivirus scanning of the Email folders in the filesystem
A antivirus scanner can slow down Thunderbird as well. Configure your antivirus program to exclude the Thunderbird Mail folders from being scanned. Warning: this option should only be used when the email is scanned by a smtp proxy of the antivirus program for viruses before it is handed to Thunderbird.
Cleanup Thunderbird index files
Thunderbird creates a lot of index files. A cleanup of these files can speedup Thunderbird as well, especially if some of them are broken. There is a handy tool called ThunderFix to do that.