This *appears* to be a Tk issue. I ran a benchmark (see attached) which repeatedly performed canvas actions. The first test ("clearing") clears the canvas, creates a rectangle, then calls both `update` and `update_idletasks`. The second test ("not clearing") is the same as the above but did not clear the canvas (causing rectangles to be drawn over each other). Here is a table of the results (times are per iteration in milliseconds):

+------------+-------------------------+-------------------------+
| Tk Version | Python                  | Wish                    |
|            +----------+--------------+----------+--------------+
|            | clearing | not clearing | clearing | not clearing |
+============+==========+==============+==========+==============+
| 8.6.8      | 6.6      | 41.4         | 4.2      | 16.2         |
+------------+----------+--------------+----------+--------------+
| 8.6.11     | 37.0     | 37.1         | 36.1     | 36.1         |
+------------+----------+--------------+----------+--------------+

A few notes:
- These were performed on a slow machine and results could be influenced by background CPU usage fluctuations
- Because it is a slow computer, I limited the iterations to just 1000 per test.
- Python version was 3.9.2
- Wish 8.6.8 was installed using an ActiveTcl binary whereas 8.6.11 was installed through homebrew