Slow inserts in EF Core after upgrading to .Net 5

I have recently upgraded a solution from .Net Core 3.1 to .Net 5. It is a simple WebAPI, that uses a MSSQL database.

Before returning a response to a request to the API, a record is saved in a table. A simple log of the request and response. It is a simple table with no foreign keys or anything. It usually takes around 5-10 ms to insert a row into this table, but after upgrading to .Net 5, I noticed that suddenly around 10% of the inserts are very slow, taking from 500-1500 ms to complete. The remaining 90% are still as fast as before.

The data is added to the database the usual way with EF Core, so no magic there:

await _dbContext.LogData.AddAsync(log);
await _dbContext.SaveChangesAsync();

After enabling debug logging in EF Core, it seems that the time is spent beginning the transaction:

Nov 4, 2021 @ 15:10:51.383  SaveChanges completed for '"ApplicationDbContext"' with 1 entities written to the database.
Nov 4, 2021 @ 15:10:51.383  An entity of type '"LogData"' tracked by '"ApplicationDbContext"' changed state from 'Added' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values.
Nov 4, 2021 @ 15:10:51.383  Disposing transaction.
Nov 4, 2021 @ 15:10:51.383  Closed connection to database '""' on server '""'.
Nov 4, 2021 @ 15:10:51.383  Closing connection to database '""' on server '""'.
Nov 4, 2021 @ 15:10:51.383  Committed transaction.
Nov 4, 2021 @ 15:10:51.376  Committing transaction.
Nov 4, 2021 @ 15:10:51.376  A data reader was disposed.
Nov 4, 2021 @ 15:10:51.376  The foreign key property '"LogData"."Id"' was detected as changed. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see property values.
Nov 4, 2021 @ 15:10:51.376  Executed DbCommand ("2"ms) 
Nov 4, 2021 @ 15:10:51.374  Executing DbCommand 
Nov 4, 2021 @ 15:10:51.374  Created DbCommand for '"ExecuteReader"' (0ms).
Nov 4, 2021 @ 15:10:51.374  Creating DbCommand for '"ExecuteReader"'.
Nov 4, 2021 @ 15:10:51.373  Began transaction with isolation level '"ReadCommitted"'.
Nov 4, 2021 @ 15:10:50.654  Beginning transaction with isolation level '"Unspecified"'.
Nov 4, 2021 @ 15:10:50.654  Opened connection to database '""' on server '""'.
Nov 4, 2021 @ 15:10:50.654  Opening connection to database '""' on server '""'.
Nov 4, 2021 @ 15:10:50.654  DetectChanges completed for '"ApplicationDbContext"'.
Nov 4, 2021 @ 15:10:50.654  DetectChanges starting for '"ApplicationDbContext"'.
Nov 4, 2021 @ 15:10:50.654  SaveChanges starting for '"ApplicationDbContext"'.
Nov 4, 2021 @ 15:10:50.654  Context '"ApplicationDbContext"' started tracking '"LogData"' entity. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values.
Nov 4, 2021 @ 15:10:50.654  '"ApplicationDbContext"' generated a temporary value for the property '"Id"."LogData"'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values.
Nov 4, 2021 @ 15:10:50.654  Entity Framework Core "5.0.10" initialized '"ApplicationDbContext"' using provider '"Microsoft.EntityFrameworkCore.SqlServer"' with options: "None"

Any idea why it would suddenly take almost a second to begin a transaction?

The .Net and EF Core version is 5.0.10, and the API is running in OpenShift in a aspnet:5.0.10-buster-slim-amd64 image using a MSSQL Server 2017 cluster. As I said, the issue didn't exist in .Net Core 3.1, but we did have a similar issue in 2.2, which we didn't find the cause for as it disappeared when we moved to 3.1

EDIT: Added more of the logging. As you can see, the entire operation takes 729 ms, and the 719 ms is spent beginning the transaction.