It's actually a very simple answer -- a 20"x20" sized piece with high dpi settings will always slow down your photoshop. When I do these sized pieces I will create them in half settings meaning make it 10" x 10" with 300 dpi. Then when you are finished with the PSD file save as it is -- when they are the layered files be sure to save them as such PSD --- So for example:
Your working PSD file should actually be from the beginning:
Depending on if you are going to be printing this work OR it is for Web will determine if you need to have it in RGB or CMYK. I will assume this is for Print work.
Save your progress as you work --- Be sure to save as (EXAMPLE: FILENAME A_PSD CMYK FULL.PSD)
When you are totally completed with your FULL file then move on to your PRINT file...
With your working file still open create a flat layer by going to the top most layer and CTRL + ALT + SHIFT + E ---- This will create a FLAT Layer at the top most in your layers palette. Then with the selection being this flat layer Select All by hitting the CTRL + A keys at the same time -- this will copy the layer then type CTRL + N , this will create a new open file - once it opens up after you confirm the settings which should be the same as above 10 W x 10 H (both in inches) and 300 dpi, click CTRL + V .... this will paste the layer that you just copied into the new file. Then use CTRL + E and this will flatten the layers down into one layer. Then save your file (Example: Filename A_JPG FLAT CMYK.JPG).
Once you have saved the file w/ the flattened layer you can now exit out of your FULL working file as you will now be working with the flattened piece that will be ready for publish (print, web, etc). Here is where you can now size it up. Keep in mind most printshops now have photoshop people working in the office to work with orders. Some can be very snooty about sizing the work back up. If you have one of these printers this is when you will want to size your work back up to the 20"x20" piece and then save. Then you are ready to send it off to the printer. If your printer is one that you can send the half size be sure to mention this when you are sending over the 1/2 sized piece so they can size it back up to where it belongs.
Hope this makes sense! Best of Luck!!