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    <p>I suspect the performance problem you are seeing is due to the block memory zeroing involved when cloning the record (plus a negligible time for allocating it and subsequently garbage collecting it) in every iteration of the loop. You could rewrite your example using a struct:</p> <pre><code>[&lt;Struct&gt;] type DeviceStatus = val RPM : int val Pressure : int val Temperature : int new(rpm:int, pres:int, temp:int) = { RPM = rpm; Pressure = pres; Temperature = temp } let rec UpdateStatusITimes (status:DeviceStatus) (i:int) = match i with | 0 -&gt; status | _ -&gt; UpdateStatusITimes (DeviceStatus(90, status.Pressure, status.Temperature)) (i - 1) let initStatus = DeviceStatus(80, 100, 70) </code></pre> <p>The performance will now be close to that of using global mutable variables or by redefining <code>UpdateStatusITimes status i</code> as <code>UpdateStatusITimes rpm pres temp i</code>. This will only work if your struct is no more than 16 bytes long as otherwise it will get copied in the same sluggish manner as the record.</p> <p>If, as you've hinted at in your comments, you intend to use this as part of a shared-memory multi-threaded design then you will need mutability at some point. Your choices are a) a shared mutable variable for each parameter b) one shared mutable variable containing a struct or c) a shared facade object containing mutable fields (like in ildjarn's answer). I would go for the last one since it is nicely encapsulated and scales beyond four int fields.</p>
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    1. POHow to make immutable F# more performant?
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    1. COThat's a bit faster, but still immutability and the inherent copying of data from the old struct/record to the new one is the bottleneck, as adding more fields to DeviceStatus causes a linear slowdown. BTW I'm not "reading" device status; the device is streaming status packets to me over serial, so I have to update my data with each new packet, which come in very fast, hence the definition of UpdateStatusWithSerialReadings.
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    2. CO@Dax. Linear in terms of the number of fields? Well, yes, but the same is true when updating global variables or function arguments. Yes, with globals you can check whether the data has changed and update it only in that case, but I would expect the read and comparison to be no faster than just a write.
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    3. COAlso, it's not copying that's the bottleneck - copying always occurs (except in the case mentioned above which is slower - I just tried). It's the allocation and garbage collection.
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