Emulator

Game Name?
Pitfall: The Lost Expedition
Pitfall: The Big Adventure

Game ID? (right click the game in the game list, Properties, Info tab)

GPHE52
RPFE52 (0001000052504645)

MD5 Hash? (right click the game in the game list, Properties, Verify tab, Verify Integrity button)

ad35176fc7a9fad11c38ec68ddcbb621
b3a92e0f816d2c2f30bf349bcbec4485 (wbfs file)

What's the problem? Describe what went wrong.

There are two retro games you can play: "Pitfall!" and "Pitfall II: Lost Caverns". Both experience very bad lag spikes, or stutters, which make them noticeably harder to play, nearly impossible to get past crocodiles. While "Pitfall II" is an optional Easter egg as a bonus for full completion, "Pitfall!" is actually a required part of the completion process for the main game.
This is most likely emulator-ception issue, as they are Atari 2600 games.

What steps will reproduce the problem?

Getting to "Pitfall!" in-game would take some time. Instead, you can unlock both minigames by entering the following codes on the title screen (where the games asks you to press start):
Pitfall!: hold L + R then press X, X, Left, Right, X, B, A, Up, X.
Lost Caverns: hold L + R then press Left, Right, Left, Right, Y, Y, Y.
Upon releasing L+R you will see the message "Cheat Code Accepted" appear on screen (see attached image).

Is the issue present in the latest stable / development version? For future reference, please also write down the version number of the latest development version.

Yes. Tested on
4.0-9017 (recommended for Pitfall:TLE),
5.0 (latest stable),
5.0-13178 (latest dev) and
5.0-13242 (latest beta)

What are your PC specifications? (CPU, GPU, Operating System, more)

[PC specs here]
CPU: AMD Ryzen 5 2600
GPU: NVIDIA GeForce GTX 1070
OS: Windows 10.0.18363 Build 18363

Is there anything else that can help developers narrow down the issue? (e.g. logs, screenshots,
configuration files, savefiles, savestates)

You can see the issue in action from 39:51 to 41:20 in this speedrun on emulator by RipperSteveM5: https://youtu.be/N2ep2hzkH7Y?t=2382 (I have of course replicated the issue on my own setup)