Code running against Outlook is very slow when PST or OST is on a network folder or non-physical/non-VHD drive.
There is no support for performance issues with Outlook where the PST or OST are on a network folder or non-physical/non-VHD drive. See below:
Limits to using personal folders (.pst) files over LAN and WAN links
https://support.microsoft.com/en-us/kb/297019
Note Customers are responsible for both defining and maintaining adequate network and disk I/O. Microsoft will not assist in troubleshooting slow performance due to networked .pst or .ost files. Microsoft will only assist if the performance issue is reproduced while the .pst or .ost file is located on either a hard disk that is physically attached to the computer that is running Outlook, or on a virtual hard disk (VHD) that is attached to the virtual machine that is running Outlook.
ImportantMicrosoft programs may not work as expected in a third-party application or software virtualization environment. We do not test Microsoft products that are running in third-party application or software virtualization environments. For more information about support provided by Microsoft for its software running together with non-Microsoft hardware virtualization software, click the following article number to view the article in the Microsoft Knowledge Base:
897615 Support policy for Microsoft software running in non-Microsoft hardware virtualization software
So, keep the above in mind when writing code which goes directly against Outlook - such as OOM or Extended MAPI code. If Outlook is performing poorly then the execution of code will also be poor. OST and PST files are for Outlooks usage and there is an immense amount of access against them - in essence they are extreme heavy usage Outlook database files. So, its critical to follow Microsoft guidelines on where its appropriate for them to exist. Over more than a decade I've seen customers trying to get decent performance and some do get performance which seems viable, however they often run into issues - especially when they mix in VPN access and come to a grinding halt - yeah, I would expect so. Another situation I've seen a number of times is where a customer is running into perf issues on virtualized servers and want to know why one has slower perf than the other - well, that's something to ask the virtualization vendor. Often such virtual servers are running different loads of applications with different loads of users - so, yes performance can and will vary.
If you really wish to test performance with code accessing Outlook then you should have Outlook and its OST & PSTs on a physical or .VHD drive. I've done a lot of testing with different types of code going against Outlook and this is the only way to get quality results.
Also see:
Does "Unsupported" Mean?
https://blogs.msdn.com/b/pcreehan/archive/2007/05/04/what-does-unsupported-mean.aspx