Worksheet functions were not designed for this. User-defined worksheet functions were not made a feature of Excel to go and fetch data online - in my humble opinion, this is a terrible abuse of UDF's.
A function like this should be part of the definition of a class module that defines an object responsible for this task, and wrapped with an interface, say ITickerPriceProvider:
Public Function GetTickerPrice(ByVal ticker As String) As Double
End Function
Then your code moves into a class module with Implements ITickerPriceProvider, and the signature becomes this:
Option Explicit
Implements ITickerPriceProvider
Private Function ITickerPriceProvider_GetTickerPrice(ByVal ticker As String) As Double
'your code
End Function
What gives? Now you can get fancy and write a macro that looks like this:
Public Sub UpdateTickerPrices()
Dim provider As ITickerPriceProvider
Set provider = New WebTickerPriceProvider
Dim updater As TickerSheetUpdater
Set updater = New TickerSheetUpdater
updater.UpdateTickerPrices provider, TickerPricesSheet
End Sub
And the TickerSheetUpdater.UpdateTickerPrices method is where you go and implement the code that locates ticker strings (your worksheet must have a column with those, right?), fetches the prices for each one, and updates the worksheet.
This is a job for a macro, not a UDF.
As a bonus, your macro can be unit-tested because the dependencies are under control - and now if somebody cuts the network cable, you can handle errors gracefully, instead of having 200 UDF calls blowing up one after another.
As for the implementation itself, I don't see any glaring issues. I would have named IE something like browser, URL would have been url, and there would have been an On Error GoTo CleanFail statement at the top, and a CleanExit label to ensure resources get cleaned up whether or not an error occurs:
Private Function ITickerPriceProvider_GetTickerPrice(ByVal ticker As String) As Double
On Error GoTo CleanFail
'implementation here
CleanExit:
Set browser = Nothing
Set pageData = Nothing
Exit Function
CleanFail:
'handle errors
Resume CleanExit
End Function
Turning ScreenUpdating off isn't going to help much, at least not with a UDF approach - the function isn't updating any cell values, it is a cell value.
However I'd try to see if there wouldn't be a way to remove this:
Application.Wait Now + TimeSerial(0, 0, 5)
...or add a comment that explains why it's needed, and why it needs to be a whole 5 seconds - especially given you've just waited for a specific ReadyState on your IE object:
Do Until IE.ReadyState >= 4
DoEvents
Loop
What's 4 anyway? Make an Enum for the possible values, and use it instead of hard-coding a value like this. Why do you need to wait 5 seconds after the browser gets into that ReadyState? Could you keep loop-waiting for a later state value? An enum would help understanding what's going on and why here.