Not saying the leftovers are right or wrong, but sometimes there is misconception about Windows registry.
The registry is huge and left over entries from uninstallation is miniscule compared to the overall picture.
Remaining entries will not cause any significant system performance issues as the registry is not accessed sequentially and the only program calling the obsolete keys is now removed from the system.
However continual removal of keys/entries will cause registry fragmentation, registry fragmentation can possibly cause performance issues if it becomes severe.
A lot of software will leave harmless registry keys behind intentionally, especially software that is updated regularly.
Reason being with the assumption that an updated version of the program will be re-installed updating the keys in the process.
Updating existing keys causes less fragmentation than removal and rewriting new keys.
Kind regards.
Edit: Spelling correction.