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Hello,
As specified by Michael_F in the comments, it is essential to identify the root cause for your latency before we jump into figuring out the solution. There could be multiple reasons for the latency. You can find out the time taken during various phases of the HTTP connection using curl as shown below.
curl -L --output /dev/null --silent --show-error --write-out 'lookup: %{time_namelookup}\nconnect: %{time_appconnect}\npretransfer: %{time_pretransfer}\nredirect: %{time_redirect}\nstarttransfer: %{time_starttransfer}\ntotal: %{time_total}\n' 'google.com'
Output for the above request looks something like below:
lookup: 0.002451
connect: 0.000000
pretransfer: 0.011611
redirect: 0.016241
starttransfer: 0.068402
total: 0.075255
Using this info, you can pin-point your request latency to a specific phase in the request-response process, and further investigate the root cause.
It is also helpful to enable ALB access logs as mentioned by Michael_F in the comments.
If you are unable to figure out the problem, please feel free to reach out to AWS Support to investigate the issue by following this link: https://docs.aws.amazon.com/awssupport/latest/user/case-management.html#creating-a-support-case
I hope this info is helpful to you!
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Before we try to dive too deep into the potential cause, let's make sure that you're looking at the right data. Instead of looking at timestamps (which might reflect the time the request was completed instead of when it was received), let's look instead at the latency data published in the ALB logs. The particular fields to look at in the ALB logs are the request_processing_time and target_processing_time fields as described here: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-access-logs.html#access-log-entry-syntax - the 6th and 7th fields.
The target_processing_time field is the most important field: "The total time elapsed (in seconds, with millisecond precision) from the time the load balancer sent the request to a target until the target started to send the response headers." Also, please compare against the actual response latency from your application logs if possible; timestamps aren't enough because they are often vague. My experience is that ALB request latency to a Fargate target is in the sub-millisecond range.