Computer lags when opening applications

Macs can run slowly for many different reasons.  These include:

• Viruses and other malware

• Corrupted user or system files

• Lack of spare disk space

• Lack of spare RAM space

• Incorrect settings

• Incompatible software

• .. and much more

It's just a case of checking everything you can. For example, run First Aid in Disk Utilities, check for malware, check disk space, reinstall macOS, etc

You didn't mention what the Mac product description line is from your Menu : About This Mac. That would guide us in the recommendation for resetting your System Management Controller (SMC). You can reset your NVRAM by pressing option+command+P+R keys from a cold boot, and letting the chime sound three times before releasing the keys.

With 8 GB RAM on Catalina, you really have only about 4GB free for launched applications, and if you run too many, your Mac will crawl because it is swapping to disk to compensate for running out of RAM.

If you are running any so-called anti-virus applications, then that would be the kiss of death for Mac performance as they are widely known to interfere with the operating system, and never find any real viruses. If this is the case, then follow the vendors explicit instructions for removal of their AV product. Don't just drag the app to the trash. Reboot.

You may have ad/malware on your Mac that is now impacting performance. Download and run the free Malwarebytes for Mac. It is not an AV tool, but is specifically engineered to detect and remove ad/malware. There is an uninstall Help menu item for Malwarebytes should you choose to remove it later.