Blueprint Editor is extremely slow on my Ubuntu 16.04. Can I improve performance without upgrading?
The poor performance in the widget editor appears to be a bug. However, keep in mind that your system does not meet the recommended specs suggested by Epic for developing using the engine;
Desktop PC or Mac Windows 7 64-bit or Mac OS X 10.9.2 or later Quad-core Intel or AMD processor, 2.5 GHz or faster NVIDIA GeForce 470 GTX or AMD Radeon 6870 HD series card or higher 8 GB RAM
Your best course of action is to file a detailed bug report with Epic to fix this specific issue, and consider upgrading for the long term.
This is a bug due to FNullSourceCodeAccessor::CanAccessSourceCode() being called too often.
There is a fix that has been applied to 4.17, and you can manually add it to your code by applying the changes found here.