[Baertram]

To see if I understood everything correct about the performance stuff I'd like to show you an example and ask, how I need to change it (including ideas).

Current code:

button:SetHandler("OnMouseEnter", function(self)
--tooltipText is a local defined variable, outside of the closure, but inside the same function which sets the handler                  
tooltipText = outputFilterState(false, settingsVars.settings.splitFilters, p_FilterPanelId, buttonId, mappingVars.settingsFilterStateToText[tostring(getSettingsIsFilterOn(buttonId, p_FilterPanelId))])
                    if tooltipText ~= "" then
                        local showToolTip = true
                        --Don't show a tooltip if the context menu for gear sets is shown at the filter button
                        if contextMenu.GearSetFilter[locVars.gFilterWhere] ~= nil then
                            showToolTip = contextMenu.GearSetFilter[locVars.gFilterWhere]:IsHidden()
                        end
                        --Don't show a tooltip if the context menu for research, deconstruction & improvement is shown at the filter button
                        if showToolTip and contextMenu.ResDecImpFilter[locVars.gFilterWhere] ~= nil then
                            showToolTip = contextMenu.ResDecImpFilter[locVars.gFilterWhere]:IsHidden()
                        end
                        if showToolTip and contextMenu.SellGuildIntFilter[locVars.gFilterWhere] ~= nil then
                            showToolTip = contextMenu.SellGuildIntFilter[locVars.gFilterWhere]:IsHidden()
                        end
                        if showToolTip then
                            ZO_Tooltips_ShowTextTooltip(button, BOTTOM, tooltipText)
                        end
                    end
                end)

Optimized code:

button.tooltipText = tooltipText
button:SetHandler("OnMouseEnter", function(self)
    button.tooltipText = outputFilterState(false, settingsVars.settings.splitFilters, p_FilterPanelId, buttonId, mappingVars.settingsFilterStateToText[tostring(getSettingsIsFilterOn(buttonId, p_FilterPanelId))])
    if self.tooltipText ~= "" then
        local showToolTip = true
        --Don't show a tooltip if the context menu for gear sets is shown at the filter button
        if contextMenu.GearSetFilter[locVars.gFilterWhere] ~= nil then
            showToolTip = contextMenu.GearSetFilter[locVars.gFilterWhere]:IsHidden()
        end
        --Don't show a tooltip if the context menu for research, deconstruction & improvement is shown at the filter button
        if showToolTip and contextMenu.ResDecImpFilter[locVars.gFilterWhere] ~= nil then
            showToolTip = contextMenu.ResDecImpFilter[locVars.gFilterWhere]:IsHidden()
        end
                    if showToolTip and contextMenu.SellGuildIntFilter[locVars.gFilterWhere] ~= nil then
                        showToolTip = contextMenu.SellGuildIntFilter[locVars.gFilterWhere]:IsHidden()
                    end
        if showToolTip then
            ZO_Tooltips_ShowTextTooltip(self, BOTTOM, self.tooltipText)
        end
    end
end)

I changed the button variable inside the anonymous SetHandler function to use the "self".
And I changed the local tooltipText variable usage inside the anonymous SetHandler function to use the self.tooltipText variable now, which was passed to the button control outside the closure.

But what about all the other variables used inside the anonymous SetHandler function, like "buttonId" (which is a parameter of the calling function, which is setting the handler), "settingsVars.settings.splitFilters" (which is an addon wide known local variable set as the settings in LAM 2.0 panel get changed), "p_FilterPanelId" (which is a parameter of the calling function too)?
Do I need to pass them to the button control too and use the self.variableName inside the anonymouse function then?