Packet trace displays that there is no issue with the controller response times, and the issue is with the client side settings.
The SMB LargeMTU feature allows an SMB client to issue a single request of up to 1MB. Without support for this feature, SMB clients (both Apple and Mac) are limited to 64KB max request size. Windows clients get around the lack of support for this feature by using another Windows feature called pipelining. Pipelining allows an SMB client to issue multiple outstanding requests without waiting for a response.
Apple supports this feature as well; however, they limit their client to just a few outstanding pipelined requests (less than 4 total outstanding requests versus Windows which regularly goes well above that). So despite a lack of support for LargeMTU, Windows clients have a better performance experience on SMB2.x than Apple because they better utilize pipelining, in place of the absence of LargeMTU with clustered Data ONTAP.
Apple Spotlight file indexer is a common cause for slowness in accessing CIFS shares.