System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1650 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 456.71

Blender Version
Broken: version: 2.90.1, branch: master, commit date: 2020-09-23 06:43, hash: rB3e85bb34d0d7 also 2.83.5

Short description of error
Rendering time increasing when Viewport Shading in Rendered mode after it stop to compute.

Exact steps for others to reproduce the error
Test scene
https://cloud.blender.org/p/gallery/5f4cd4441b96699a87f70fcb

0. Make sure you use only GPU in Pref - System - CUDA

1. Switch Render Engine to Cycles - GPU Compute
2. Change View to Solid this time and hit F12.
3. Remember rendering time.
4. Change View to Rendered.
5. Wait until it stop to compute in viewport and then
6. Hit F12 and watch how rendering become slow…

Results:
2.83.5 - F12 from Solid View - Rendering Time - 3:29

2.83.5 - F12 from Rendered View (after it stop to compute) - Rendering Time - 5:47

2.90.1 - F12 from Solid View - Rendering Time - 3:28

2.90.1 - F12 from Rendered View (after it stop to compute) - Rendering Time - 5:46

if you already know about this issue, you can make a checking when pressing F12, if in Solid view then render, if Rendered view then go to Solid and render.

In some cases, I don't know yet how to reproduce, rendering time become insane like this